Effect of Ozonation and Chlorination on Humic Substances in Water


Dec 15, 1988 - Ozonation converts humic substances, as can be seen by a small decrease in dissolved organic carbon (DOC) and a substantial decrease in...
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Effect of Ozonation and Chlorination on Humic Substances in Water Joop C. Kruithof, Marten A. van der Gaag, and Dick van der Kooy Kiwa Ltd., P.O. Box 1072, 3430 BB Nieuwegein, Netherlands

Ozonation converts humic substances, as can be seen by a small decrease in dissolved organic carbon (DOC) and a substantial decrease in UV extinction. This conversion is related to the formation of low-molecular-weight biodegradable compounds, which enhance regrowth of organic substances in water during distribution. Posttreatment by coagulation andfiltrationprocesses can remove these types of compounds. Chlorination of humic substances causes the production of trihalomethanes (THMs), high-molecular-weight organohalides, and mutagenicity. A partial THM-precursor removal by pretreatment does not reduce the THM content under practical conditions and causes a shift to production of more highly brominated THMs. Extensive pretreatment with ozonation and granular activated carbon (GAC)filtrationlowers the adsorbable organohalogen (AOX) content and the mutagenic activity in the Ames test.

WATER SOURCES USED FOR DRINKINGW - ATER PREPARATO IN contain humic substances, which account for roughly 75% of the total dissolved organic carbon (DOC). These humic substances strongly interfere with water-treat­ ment processes, especially in surface-water treatment. Therefore, the Neth­ erlands Waterworks Testing and Research Institute Kiwa Ltd. and the Netherlands Waterworks have been carrying out investigations into the re­ moval and conversion of bulk organic materials (mainly humic substances) by water-treatment processes.

0065-2393/89/0219-0663$06.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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AQUATIC H U M I C SUBSTANCES

Many analytical methods are available for a general characterization of humic substances in water: determination of D O C content as a measure of the concentration of humic substances; spectrophotometric analysis (meas­ urement of U V extinction and color) as a measure of the concentration and, in combination with the D O C content, as a first indication of the character of the humic substances; and gel permeation chromatography and X-ray scattering to determine the molecular-size distribution. Other methods are used to assess the interference of humic substances with treatment processes. Examples of these methods are determination of easily assimilable organic carbon (AOC) content as a measure of the regrowth potential of the organic materials, especially after oxidative treatment, and determination of trihalo­ methane ( T H M ) precursors as a measure of the potential T H M formation upon chlorination. A l l of these parameters have been used to characterize the concentration and nature of humic substances during Dutch drinking-water treatment. For this chapter we will concentrate on • D O C content as a measure of the content of humic substances •

UV254 extinction at a wavelength of 254 nm as an indication of the changing character of the humic substances during oxidative treatment

• A O C content as a measure of the regrowth potential of the water • THM-precursor content as a measure of potential T H M for­ mation upon chlorination. Treatment processes influencing the concentration of humic substances are coagulation and granular activated carbon (GAC) filtration. Standardized jar test equipment was developed (1) to study the effect of coagulation on the content of natural organic compounds. G A C filtration is applied at 12 full-scale plants, primarily to remove toxic compounds and to improve taste (2, 3). Removal of humic substances (removal of color, U V extinction, and dissolved organic carbon) is an important secondary aim. This chapter will consider the interaction of oxidative processes with humic substances in water. The processes studied most extensively are ozon­ ation and chlorination. Data on oxidation of humic substances by ozonation are available from 12 pilot-scale and eight full-scale experiments (4). Ozonation of humic sub­ stances leads to the production of low-molecular-weight compounds, which increase the availability for bacteria of organic compounds in water. This phenomenon is responsible for the regrowth of bacteria during distribution (5). Removal of these biodegradable compounds by posttreatment of the ozonated water is necessary to restrict bacterial regrowth during distribution

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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Effect of Ozonation and Chlonnation

(6). The effects of ozonation will be illustrated through D O C , U V extinction, and A O C measurements gathered from pilot-scale and full-scale experiments carried out at the treatment plants at Driemond (Municipal Waterworks of Amsterdam) and at Kralingen (Waterworks of Rotterdam). In the years since the discovery of T H M production during drinkingwater chlorination (7, 8), measures have been taken to reduce this side effect of chlorination as much as possible (9). One option was the removal of humic substances by G A C filtration before chlorination. Under Dutch chlorination conditions, partial removal of humic substances by G A C filtration did not give a substantial reduction of the T H M content for long GAC-filter running times. Moreover, chlorination of GAC-filter effluents caused a shift to the production of more highly brominated T H M s , especially at short GAC-filter running times (10-12). In addition to a relatively high adsorbable organic halogen (AOX) con­ tent, mutagenic activity was found after postchlorination with a relatively low chlorine dosage of about 0.5 m g / L (13, 14). Both side effects of chlori­ nation will be illustrated on the basis of T H M and THM-precursor meas­ urements gathered from full-scale experiments carried out at the treatment plant at Zevenbergen (Waterworks of North West Brabant) and on the basis of A O X and mutagenic activity measurements carried out at the pilot plant at Nieuwegein (Kiwa Ltd).

Effects and Side Effects of Ozonation Experimental Parameters. An impression of the character and the content of humic substances in the water was obtained by determination of the UV extinction and the DOC content. In samples of nonozonated and ozonated water, the UV extinction was determined by spectrophotometer (Perkin Elmer, Type 500S) at a wavelength of 254 nm. The DOC content was determined subsequently, after acidification to pH 2 with concentrated hydrochloric acid and membrane filtration, with an ultra-low-level organic analyzer system (Dohrmann DC-54). To determine the concentration of AOC, 600 mL of water was heated in thor­ oughly cleaned Pyrex Erlenmeyer flasks at 60 °C for 30 min to inactivate the bacteria orginally present in the water. After cooling, pure cultures of selected bacteria were inoculated into the samples, which were incubated at 15 °C. Growth of these bacteria in the water samples was measured by periodic determinations of the number of viable organisms. The maximum colony count is considered a measure of the amount of AOC available for the organism in the water used in the growth experiment. Two bacterial strains were used: Pseudomonasfluorescensstrain P17, able to metabolize a great variety of organic compounds such as amino acids, carbohydrates, and aromatic acids: AOC (P17); and Spirillum species strain NOX, specialized in the use of carboxylic acids such as formic acid, glyoxylic acid, and oxalic acid: Δ AOC (NOX). Yield values of P17 for acetate and of NOX for oxalate have been determined for use in calculation of the AOC concentration. The total AOC (AOC ) is AOC = AOC (P17) + Δ AOC (NOX). In this equation, AOC (P17) is expressed in micrograms of acetic acid carbon (Ac C) per liter and Δ AOC (NOX) is expressed in micrograms of oxalic acid carbon (Ox C) per liter. Detailed information about the determination has been published elsewhere (6, 15, 16). T

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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AQUATIC H U M I C SUBSTANCES

Application of Ozonation. In the Netherlands ozone is applied at eight full-scale treatment plants. The two largest plants are the Driemond plant of the Municipal Waterworks of Amsterdam with a production of 22 Χ 10 kL/year and the Kralingen plant of the Waterworks of Rotterdam with a production of 33 X 10 kL/year. The treatment systems of these plants are shown in Figures 1 and 2. The average ozone dosage at both plants is about 2.5-2.7 m g / L . Pilot-scale ozonation experiments have been carried out with rapid fil­ trate from lake water at the Driemond plant and surface water after coag­ ulation at the Kralingen plant. The effect of the ozonation on humic substances is expressed as the reduction of the U V extinction and D O C as a function of the ozone dosage (mg of 0 / m g of D O C ) . The side effect of the ozonation on humic substances is expressed as the A O C = A O C (P17) + Δ A O C (NOX) fraction of the D O C content as a function of the ozone dosage. Full-scale experiments were carried out to investigate the variation in A O C (P17) from raw to finished water at both treatment plants. In addition, full-scale data were gathered about the formation of A O C during the ozon­ ation step. 6

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Effect of Ozonation on Humic Substances.

The reduction of the

U V extinction as a function of the ozone dosage is presented in Figure 3. At a maximum ozone dosage of about 2.0 mg of 0 / m g of D O C , the U V 3

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Figure 1. Treatment system at Driemond, Municipal Waterworks ofAmsterdam.

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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storage

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Figure 2. Treatment system at Kralingen, Waterworks of Rotterdam.

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Figure 3. Reduction of UV extinction by ozonation; pilot-scale experiments at Driemond and Kralingen.

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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extinction was reduced by about 60-70% for both water types. About 40% of the initially present UV-absorbing compounds showed the greatest sus­ ceptibility for ozonation at ozone dosages up to 0.5 mg of 0 / m g of D O C . This conversion of UV-absorbing compounds is reached under practical con­ ditions. The reduction of the D O C content is given in Figure 4. At maximum ozone dosage, the D O C at Kralingen was reduced by about 8%. At Driemond a maximum conversion of 18% could be reached. Under practical ozonation conditions, these percentages proved to be about 4% and 8%, respectively. These experiments indicate that for the maximum ozone dosage the U V extinction is reduced 60-70% and the D O C removal 8-18%. This prefer­ ential decrease of the 254-nm chromophore indicates that ozonation causes a change in the character of the humic substances, without a complete oxidation of the organic compounds to carbon dioxide. This conversion of humic substances is the cause of the increased concentration of biodegradable compounds.

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AOC Formation. For both water types, pilot-scale experiments showed an increase of A O C as a function of the ozone dosage. A t the maximum ozone dosage A O C amounted to 550 μ g / L for ozonated rapid filtrate at Driemond and 800 μ g / L for ozonated coagulated water at Kral­ ingen. The A O C / D O C ratio as a function of the ozone dosage is presented in Figure 5. For both water types the A O C / D O C ratio showed a gradual rise. The values for ozonated water at Kralingen were much higher than those at Driemond (0.3 and 0.09, respectively). These data indicate that the T

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